1. Field of the Invention
The present invention is directed to a discharge circuit for use in a multielectrode defibrillator or cardioverting device and methods of using such a circuit.
2. Background
Cardiac devices for treating electrical malfunctions of a heart are known. These devices operate by delivering an electrical shock to the heart, which typically stimulates the heart and causes it to begin beating normally. Various devices have been developed over the years for treating a variety of different malfunctions. Some such devices are designed to monitor the heart and deliver a therapeutic shock automatically upon detection of a malfunction. Other devices operate manually, delivering a shock only upon intervention by a user.
One cardiac device is a defibrillator. A defibrillator delivers a relatively large electric shock to a heart that is in fibrillation. Early defibrillators delivered a monophasic shock to the heart. In a monophasic shock, the polarity of the voltage remains the same throughout the shock. It was soon discovered, however, that a biphasic shock can be more effective in treating fibrillation than a monophasic shock. In a biphasic shock, a first portion of the waveform applied to the heart has a first polarity and a second portion of the waveform has an opposite polarity. Typically, a biphasic shock is able to defibrillate a heart with less energy and/or voltage than a monophasic shock.
Known circuits for delivering a biphasic shock utilize either a single discharge capacitor or two discharge capacitors configured to act as a single capacitor. An example of the former circuit is shown in FIG. 1. As shown in FIG. 1, four switches 104, 106, 114, and 116 control the discharge of capacitor 102 through electrodes 108 and 112 into heart 110. Upon determination that a defibrillating shock needs to be delivered to heart 110, a charging circuit (not shown) charges capacitor 102. When capacitor 102 is sufficiently charged, switches 104 and 116 are closed, while switches 106 and 114 remain open. Capacitor 102 begins to discharge, creating a current that flows from electrode 108 to electrode 112. When capacitor 102 has only partially discharged, switches 104 and 116 are opened. Shortly thereafter, switches 106 and 114 are closed. Capacitor 102 then continues to discharge, but this time current flows from electrode 112 to electrode 108. The circuit shown in FIG. 1 is commonly referred to as an xe2x80x9cHxe2x80x9d bridge.
FIG. 2 illustrates an example of a prior art circuit in which two capacitors arranged to act as a single capacitor create a biphasic shock. In the circuit of FIG. 2, four switches 206, 208, 216, and 218 control discharge of two capacitors 202 and 204 through electrodes 210 and 214 into heart 212. As with the circuit of FIG. 1, a charging circuit (not shown) charges capacitors 202 and 204. When these capacitors are sufficiently charged, switches 206 and 218 are closed, while switches 208 and 216 remain open. Capacitors 202 and 204, in series, begin to discharge, creating a current that flows from electrode 210 to electrode 214. When these capacitors have only partially discharged, switches 206 and 218 are opened. Shortly thereafter, switches 208 and 216 are closed. With switches 208 and 216 closed and switches 206 and 218 open, capacitor 204xe2x80x94but not capacitor 202xe2x80x94is charges. This creates a current that flows from electrode 214 to electrode 210.
The timing of the opening and the closing of the switches in FIGS. 1 and 2 is typically controlled using one of three general methods. The first is known as the fixed tilt method. Switches 104 and 116 or switches 206 and 218 are closed until the voltage on capacitor 102 or capacitors 202 and 204 falls below a predetermined level. Once the voltage on these capacitors falls to the predetermined level, switches 104 and 116 or switches 206 and 218 are opened. The second general method of controlling the switches is known as the fixed duration method. Switches 104 and 116 or switches 206 and 218 are closed for a predetermined period of time. Once the predetermined period of time expires, the switches are opened. The third general method of controlling the timing of the opening and closing of the switches is a hybrid of the fixed tilt method and the fixed duration method. Switches 104 and 116 or switches 206 and 218 are closed for a predetermined period of time that begins when the voltage on capacitor 102 or capacitors 202 and 204 falls below a predetermined level.
Because of the high voltages and currents required to defibrillate a heart, the switches in circuits such as those shown in FIGS. 1 and 2 must be rugged. In particular, they must be capable of xe2x80x9chot switchingxe2x80x9d, i.e., closing and opening when there is a high voltage potential across the switch. Examples of switches that have been used in prior art devices include, but are not limited to, metal oxide semiconductor field effect transistors, insulated gate field effect transistors, insulated gate bipolar transistors, and silicon controlled rectifiers.
The present invention is directed to a discharge circuit for use in a multielectrode defibrillator or cardioverting device and methods of using such a circuit. The circuit includes at least two electrodes that are in electrical contact with a heart. Upon determination that a therapeutic shock needs to be applied to the heart, two capacitors configured for independent discharge are charged. Once these capacitors are sufficiently charged, one of the capacitors is switched such that it begins to discharge into the heart. At an appropriate time, this capacitor is switched again such that it no longer discharges into the heart. At this time, the other capacitor is switched such that it begins to discharge into the heart, and at the appropriate time, it is switched such that it stops discharging into the heart. The two capacitors are preferably configured with opposite polarities so that the waveform applied to the heart by the sequential discharging of the two capacitors is biphasic.